There is an X-ray computed tomography apparatus (to be referred to as a photon counting X-ray CT apparatus hereinafter) that reconstructs an image regarding an object based on the number of photons of X-rays in an output from an X-ray detector. Performance requested of the X-ray detector of the photon counting X-ray CT apparatus is a high counting rate (the ratio of the number of detected photons to the number of photons entering the X-ray detector). The high counting rate is, e.g., a counting rate of about 10^6 per 1 mm^2. For example, a semiconductor detector of CZT (CdZnTe: cadmium zinc telluride) or CdTe (cadmium telluride) has been prototyped as the X-ray detector. In order to obtain a high counting rate, the semiconductor detector is devised to decrease the light receiving area of each or a plurality of semiconductor elements in the semiconductor detector.
However, the semiconductor detector causes electrical polarization owing to repetitive incidence of X-ray photons. This puts a problem that the performance of the semiconductor detector changes.
Another X-ray photon detection method is a method of combining a scintillator and a photodetector. The method of combining a scintillator and a photodetector is free from the polarization problem, unlike the semiconductor detector. However, this method has a problem that output electrical signals overlap each other owing to generation of an incidence event by another X-ray photon during the emission time (typically about 40×10−9 s) of the scintillator. In addition, when the next X-ray photon enters during charging of the photodetector, an output signal becomes weaker than an output signal upon incidence of a single X-ray photon, failing in obtaining a proper energy integrated value. These problems are specific to the method of combining a scintillator and a photodetector, and are called pile-up.